1. Field of the Invention
The present invention relates to disk drive systems and, more specifically, to an apparatus and method for controlling the spindle speed to compensate for drag on the spindle motor.
2. Description of the Related Art
In a disk drive system, the disks are connected to a spindle which is rotated by a spindle motor. The transducer heads are loaded onto an air bearing which is caused by the rotation of the disks. The transducer's height above the disk is a function of the downward force produced by the spring tension of the head arm assembly in opposition to the upward forces caused by the air bearing itself. It is desired to fly the transducer as close as possible to the disk surface without physical contact with the disk to prevent damage to the disk and the transducer.
As technology has improved, the flying height of the transducer above the disk has been decreased to the point where the use of air as the media for the interface between the transducer and the disk is the limiting factor. The distance that the transducer flies above the disk is a factor that determines the track density that can be used on the disk and, therefore, how much data can be stored on the disk.
Attempts have been made to use other substances to coat the disk surface and to be the interface between the transducer and the disk. These attempts have identified viscous substances for coating a disc which will allow the transducer to fly closer to the disk surface than using air as the interface substance between the transducer and the disk.
When viscous substances are used a non uniform drag profile across the useable area of the disk is experienced. This non uniform drag profile is caused by the transducers interacting with the viscous substances to produce a drag on the disk which places a load upon the spindle motor which is greater than when air is used as the interface substance. Further, the magnitude of the drag increases from the inner diameter to the outer diameter of the recording area on the disk. The magnitude of the drag is also a function of the environment of the viscous substance whose viscosity will vary as a function of temperature and humidity on the recording media.
It has been found that under some circumstances, such as a light weight disk having low inertia, that the drag profile of an air bearing may be sufficiently non uniform, across the useable disk surface, to cause the same problem experienced with the use of viscous substances to coat the disk.
Disk drive systems employ a sector servo system to control the placing and maintaining of the transducers at specific tracks on the disk. The sector servo system requires each track to be divided into a fixed number of sectors and information is recorded at the beginning of each sector, including a sector mark indicating the start of the sector, servo data used to control the position of the transducer with respect to the center of the track, and identification data which identifies the track and sector.
Many disk drive systems used the recorded sector mark to determine the rotational velocity of the disk and to adjust the speed of the spindle motor such that the measured rotational velocity is maintained at a specified operational rotational velocity. The rotational speed of the disks is determined by measuring the time between detection of adjacent sector marks on a track by the spindle speed control system.
When the disk drive system performs a seek operation the transducers are moved from one track to another track on the disk. A seek algorithm is used to control the movement of the transducers from the present track to the newly designated track. Basically, the seek operation counts the number of tracks that are being crossed by the magnetic transducer as it moves across the recording surface of the recording media by sensing the servo information recorded on each track. When the transducer arrives at the new track, synchronization is obtained by use of the sector marks, the servo data and the identification data track. The track seeking procedure may employ a table of ideal velocity curves for efficiently controlling the time required for the actuator to move the transducer to the desired location. Each ideal velocity curve represents the ideal velocity profile for the actuator for movement of the transducer over a specific distance, the number of tracks. The system selects the ideal velocity curve which best matches the number of tracks to be crossed in the seek operation.
When a seek operation is performed in a disk drive systems having a non uniform drag profile, such as those employing viscous substances to coat the disks, the magnitude of the change in the drag experienced by the spindle motor can be such that the speed of the spindle motor will either increase or decrease to such a degree that the spindle speed control system cannot recover from the change in speed. This condition prevents the disk drive system from successfully performing a long seek operation.